Answer: Amebic pneumonia; most likely due to Acanthamoeba spp. or Balamuthia mandrillaris.
This is a rare manifestation of disease with the free-living amebae. More commonly, Acanthamoeba spp. cause amebic keratitis - typically in contact lens wearers. However, this organism and B. mandrillaris can rarely gain access to the systemic circulation of immunocompromised (and occasionally immunocompetent) individuals and travel to the brain, where it causes granulomatous amebic encephalitis. The lungs and skin are thought to be primary sites of entry into the body.
These amebae can be differentiated from Entamoeba histolytica by the presence of a large karyosome and cysts in tissue. In comparison, E. histolytica has a small central karyosome with uneven rim of clumped chromatin, and it does not form cysts in tissue.
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